Expanded polypropylene resin article are used in various fields such as wrapping materials, shock-absorbers, heat insulators, construction materials, etc. because the expanded polypropylene resin article is excellent in shock absorbing property, heat insulating property, etc. Especially, an in-mold foaming for producing a foamed product in a predetermined shape by introducing pre-expanded polypropylene resin beads into a mold and then heating the pre-expanded polypropylene resin beads in the mold, for example by steam, so as to fuse the pre-expanded beads with each other, is employed in various applications because the in-mold foaming can produce products of complicate shapes relatively easily.
One of the applications is cushioning packaging materials for electronic parts and mechanical parts of OA (office automation) apparatuses. In some cases, the cushioning packaging materials for electronic parts and mechanical parts should be dust-free and electrostatic-free. Therefore, in-mold foamed products produced from the pre-expanded polypropylene resin beads having antistatic property are used for this application.
Typical and general examples of methods for providing an in-mold foamed product with such an antistatic property are to apply a surfactant on a surface of the in-mold foamed product, or producing the in-mold foamed product from a resin in which a surfactant has been mixed in.
It is well known that the latter method, which the in-mold foamed product from a resin in which a surfactant has been mixed in, can provide a more persistent antistatic property to the in-mold foamed product and can be carried out with a simpler process, compared with the former method in which a surfactant is applied on the surface of the in-mold foamed product.
Patent Literature 1 has disclosed an art in which non-crosslinked polyolefin resin, as a base resin, containing a nonionic surfactant having an antistatic property and an average molecular weight of 200 to 1000 by 0.1 wt % to 5 wt % is used as pre-expanded polypropylene resin beads with an antistatic property.
Moreover, Patent Literature 2 has disclosed an art regarding pre-expanded polypropylene resin beads in which a nonionic surfactant having an antistatic property and an average molecular weight of 200 to 1000 by 0.1 wt % to 5 wt % is contained and a DSC curve obtained by differential scanning calorimetry shows a high temperature peak of a calorie of 10 J/g to 30 J/g.
Patent Literatures 1 and 2 state that the use of the particular nonionic surfactants in their techniques makes it possible to produce in-mold foamed products having necessary antistatic properties without causing a large amount of bleeding of the surfactants having the antistatic property. Further Patent Literatures 1 and 2 exemplify glycerin esters of higher fatty acids, and the like surfactants as examples of such nonionic surfactants.
Glycerin esters of higher fatty acids are relatively safer than other surfactants, and therefore are popularly used as nonionic surfactants. Such glycerinesters of higher fatty acids can be classified into monoesters, diesters, and triesters, depending on bonding types of the higher fatty acids. However, Patent Literatures 1 and 2 disclose monoesters as the glycerin esters of higher fatty acids.
The pre-expanded beads for use in the in-mold foaming are generally produced by (i) introducing resin pellets, an aqueous dispersion solvent, dispersion agent, a dispersing auxiliary agent, a foaming agent and/or the like in a pressure-resistant vessel, (ii) heating them in the pressure-resistant vessel under pressure so as to impregnate the foaming agent into the resin pellets, and (iii) releasing the resin pellets into a low pressure atmosphere (hereinafter, this method may be referred to as “depressurization foaming”). The dispersion agent is added for preventing the resin pellets from adhering with each other in the pressure-resistant vessel. However, if there is a residual dispersion agent on the pre-expanded beads, the residual dispersion agent retards the fusion of the pre-expanded beads at the in-mold foaming. Therefore, it is desirable that an amount of the residual dispersion agent on the surfaces of the pre-expanded beads is as small as possible. In view of this, for example, a step of washing the pre-expanded beads after the pre-expansion is required.
There is a tendency that it is more difficult to wash off the dispersion agent from the pre-expanded polypropylene resin beads containing a surfactant for its antistatic property, than from those not containing such surfactant. For example, Patent Literature 3 discloses a technique for carrying out the washing with, for example, 2N nitric acid solution so as to reduce the amount of the residual dispersion agent to 0.2 part, by weight or less per 100 parts by weight of the pre-expanded beads.
It is also known to add a mixture of glycerol monoglyceride and glycerol diglyceride as a permeability-adjusting additive, to shrunk thermoplastic polymer foaming beads (see Patent Literature 4). The permeability-adjusting additive is an additive for adjusting permeability of a foaming agent contained in the thermoplastic polymer foamed beads obtained by extrusion foaming. Therefore, what is disclosed in Patent Literature 4 is not a technique regarding thermoplastic polymer foamed beads obtained by depressurization foaming. Therefore, Patent Literature 4 is silent about how the permeability-adjusting additive affects the amount of the residual dispersion agent adhered on surfaces of the thermoplastic polymer foamed beads (pre-expanded beads).
Apart from polypropylene resin, Patent Literature 5 discloses a technique regarding a pre-expanded polyethylene resin beads containing a fatty acid glycerin ester having a HLB value of not less than 3 but less than 4, and a inciting point of not lower than 35° C. but not higher than 75° C., and an in-mold foamed product produced from the pre-expanded ethylene resin beads. Patent Literature 5 describes that a mixture of monostearate glycerin ester and distearate glycerin ester is used as the fatty acid glycerin ester. However, Patent Literature 5 is silent about a ratio between monostearate glycerin ester and distearate glycerin ester. Further, Patent Literature 5 describes that resultant pre-expanded foamed particles are washed with metaphosphate soda of 30 ppm concentration. This implies that it is difficult to wash off the dispersion agent from the surfaces of the pre-expanded beads even in this case where a glycerin ester of a fatty acid is used as an antistatic agent.
As described so far, there has been no art for obtaining pre-expanded polypropylene resin beads having a good antistatic property with no large amount of the residual dispersion agent on the surfaces of the pre-expanded polypropylene resin beads without requiring washing with a chemical such as nitric acid, phosphate soda or the like.